Secondary or rechargeable lithium-ion batteries may be used as electric energy storage systems for powering electric and hybrid electric vehicles. These batteries comprise a plurality of suitably interconnected electrochemical cells each of which undergoes a specific chemical reaction capable of generating electrical energy. When suitably arranged, these cells provide a predetermined electrical current at a specified electrical potential to an external load, such as an electric motor. Once discharged, such a battery may be re-charged by supplying electrical energy to the battery to reverse the chemical reaction undergone at the electrodes and render the battery again capable of delivering electrical power.
In each cell of a lithium battery, lithium is transported as lithium ions from a negative electrode through a non-aqueous, lithium-containing, electrolyte solution to a lithium ion-accepting positive electrode as an electric current is delivered from the battery to an external load, for example, in a vehicle, an electric traction motor. A suitable porous separator material, infiltrated with the electrolyte solution and permeable to the transport of lithium ions in the electrolyte, is employed to prevent short-circuiting physical contact between the electrodes.
Graphite has been commonly used as a negative electrode material in such batteries and is generally employed as a thin particulate layer bonded to a copper current collector. During charging of the cells, lithium is inserted into the graphite (lithiation), forming LiC6, with a capacity of about 372 mAh/g, and extracted from the graphitic carbon during discharging (de-lithiation).
A suitable particulate material for receiving and storing inserted lithium during discharge of each cell is used as the positive electrode material. Examples of such positive electrode materials include lithium cobalt oxide (LiCoO2), a spinel lithium transition metal oxide such as spinel lithium manganese oxide (LiMnxOy), a lithium polyanion such as a nickel-manganese-cobalt oxide [Li(NixMnyCoz)O2], lithium iron phosphate (LiFePO4), or lithium fluorophosphate (Li2FePO4F), or a mixture of any of these materials. Suitable positive electrode materials are often bonded as a thin layer to an aluminum current collector. The electrochemical potential of such lithium ion cells is typically in the range of about 2 to 4.5 volts.
The use of lithium-ion batteries to power electric motors in automotive vehicles has led to the need for higher gravimetric and/or volumetric capacity batteries. While graphitic carbon is a durable and useful lithium-intercalating, negative electrode material for lithium-ion cells, it has a relatively low capacity (372 mAh/g) for such lithium insertion. Other potential negative electrode materials such as silicon (theoretical capacity, about 3600 mAh/g for Li15Si4) and tin (theoretical capacity, about 990 mAh/g for Li22Sn5) have much higher theoretical capacities than graphite for lithium insertion.
However, unlike graphite, silicon undergoes a volume change of up to 300 volume percent during the course of lithiation and de-lithiation. Tin exhibits similar behavior. Such dramatic volume changes may induce appreciable stresses which may lead to fracture of the active (for example, silicon or tin) material so that a broken-off portion of the active material (for example, silicon or tin) loses electrical contact with the remainder of the electrode. Any portion of the electrode material not in electrical contact with the remainder of the electrode material may not participate in the electrochemical reactions of the battery and so reduces battery capacity on subsequent charge-discharge cycling. Obviously any such factures which result in separation of the electrode from the current collector will likewise reduce battery capacity on charge-discharge cycling, a phenomenon commonly known as battery fade.
Thus there remains a need for a more effective way of utilizing high energy capacity negative electrode materials such as silicon or tin to enable development of a high-capacity, fade resistant lithium ion battery.